chore: add knip config for dead-code detection

knip.json declares the real entry points (scripts, tests, Remotion roots)
and the devDeps invoked only as external CLIs (esbuild for build,
agent-browser/remotion via npx) so future scans surface only true
findings.

Add \`npm run knip\` as the canonical invocation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -27,6 +27,7 @@
     "changeset": "changeset",
     "version-packages": "changeset version && npm install --package-lock-only && node scripts/check-lockfile-sync.mjs",
     "check:lockfile": "node scripts/check-lockfile-sync.mjs",
+    "knip": "npx --yes knip@latest",
     "release": "changeset publish"
   },
   "workspaces": [